MEMS device

ABSTRACT

A MEMS device includes a backplate electrode and a membrane disposed spaced apart from the backplate electrode. The membrane includes a displaceable portion and a fixed portion. The backplate electrode and the membrane are arranged such that an overlapping area of the fixed portion of the membrane with the backplate electrode is less than maximum overlapping.

This application claims the benefit of U.S. provisional application No.61/870,112 filed Aug. 26, 2013, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

Embodiments of the invention refer to a MEMS device, to an electrostatictransducer and to a method for manufacturing a MEMS device.

BACKGROUND

A double backplate microphone in MEMS device technology comprises a topbackplate electrode and a bottom backplate electrode being arranged inparallel to each other, and a membrane being disposed between the topbackplate electrode and bottom backplate electrode in parallel. The topbackplate electrode, bottom backplate electrode and membrane aresupported by a support structure. This arrangement is supported by asubstrate.

In order to transmit sound pressure waves, for example speech, to themembrane interposed between the top backplate electrode and bottombackplate electrode, these electrodes may be perforated. Sound pressurewaves cause the membrane to vibrate due to a pressure difference overboth planes of the membrane. Hence, the air gap between the membrane andeach of the backplate electrodes varies. The backplate electrodes andthe membrane may comprise electrically conductive material(s). Thevariation of the membrane in relation to the backplate electrodes causesvariation in the capacitances between the membrane and the bottombackplate electrode as well as between the membrane and the topbackplate electrode. This variation in the capacitances is transformedinto an output signal responsive to the movement of the membrane. Themembrane may be biased by a bias voltage relative to the bottombackplate electrode and the top backplate electrode.

The double backplate microphone as described above schematically,suffers from parasitic capacitances, created inside the supportstructure. A first parasitic capacitance can be created inside thesupport structure between the membrane and the top backplate electrode.A second parasitic capacitance can be created inside the supportstructure between the membrane and the bottom backplate electrode. Athird parasitic capacitance can be created inside the support structurebetween the bottom backplate electrode and the substrate. The substratecan be grounded. In other words, parasitic capacitances tend to becreated between the top backplate electrode, membrane and bottombackplate electrode in combination, inside the support structure, i.e.,in portions of the MEMS device excluding the air gap between themembrane and the top backplate electrode, as well as the air gap betweenthe membrane and the bottom backplate electrode.

Parasitic capacitances are usually unwanted capacitances interferingwith capacitances between both the membrane and the top backplateelectrode as well as between the membrane and the bottom backplateelectrode. Hence, capacitance values, intended to be transformed intoelectrical signals responsive to the movement of the membrane areinterfered. In case the MEMS device is embodied as a double backplatemicrophone, for example, parasitic capacitances may influence the MEMSdevice such that the (electrical) output does not correspond to acorrect reproduction of the (audible) input. While not mentioned,further sources of parasitic capacitances are conceivable.

SUMMARY OF THE INVENTION

An embodiment of the invention provides a MEMS device comprising abackplate electrode. A membrane is disposed spaced apart from thebackplate electrode. The membrane comprises a displaceable portion and afixed portion. The backplate electrode and the membrane are arrangedsuch that an overlapping area of the fixed portion of the membrane withthe backplate electrode is less than maximum overlapping.

A further embodiment provides an electrostatic transducer comprising abackplate electrode. A membrane is disposed spaced apart from thebackplate electrode. The membrane comprises a displaceable portion and afixed portion. The backplate electrode and the membrane are arrangedsuch that an overlapping area of the fixed portion of the membrane withthe backplate electrode is less than maximum overlapping. Theelectrostatic transducer is configured to produce an output signalresponsive to a movement of the membrane in relation to the backplateelectrode.

A further embodiment provides a method for manufacturing a MEMS devicethat comprises a backplate. A membrane is disposed spaced apart from thebackplate electrode, wherein the membrane comprises a displaceableportion and a fixed portion. The method comprises providing thebackplate electrode and the membrane such that an overlapping area ofthe fixed portion of the membrane with the backplate electrode is lessthan maximum overlapping.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described in the following withrespect to the figures.

FIG. 1a shows a schematic MEMS device in cross-section;

FIG. 1b shows a schematic circuit diagram of the MEMS device depicted inFIG. 1 a;

FIG. 2a shows a further schematic MEMS device in cross-section;

FIG. 2b shows a schematic circuit diagram of the MEMS device depicted inFIG. 2 a;

FIG. 3a shows a plan view of a MEMS device;

FIG. 3b shows a schematic cross-sectional view of the MEMS devicedepicted in FIG. 3 a;

FIG. 4a shows a further plan view of a MEMS device;

FIG. 4b shows a schematic cross-sectional view of the MEMS devicedepicted in FIG. 4 a;

FIG. 5a shows a further plan view of a MEMS device;

FIG. 5b shows a schematic cross-sectional view of the MEMS devicedepicted in FIG. 5 a;

FIG. 6 shows a schematic diagram of a MEMS device in a cross-sectionalview;

FIG. 7 shows a schematic plan view of a backplate electrode of the MEMSdevice;

FIG. 8a shows a further plan view of a MEMS device;

FIG. 8b shows a schematic cross-sectional view of the MEMS devicedepicted in FIG. 8 a;

FIG. 8c shows in a schematic diagram a plan view of the arrangement of atop backplate electrode, a membrane and a bottom backplate electrode inrelation to each other;

FIG. 9a shows in a cross-sectional view a schematic diagram of a MEMSdevice comprising a guard ring;

FIG. 9b shows in a schematic diagram a plan view of the arrangement of atop backplate electrode, a membrane, a bottom backplate electrode andassociated guard rings in relation to each other; and

FIGS. 10a-10p schematically illustrate a process flow of a method formanufacturing the MEMS device.

Different embodiments of the teachings disclosed herein willsubsequently be discussed referring to FIG. 1 to FIG. 10p . In thedrawings, identical reference numerals are provided to objects havingidentical or similar functions so that objects referred to by identicalreference numerals within the different embodiments are interchangeableand the description is mutually applicable.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring to FIG. 1a , a MEMS device 10 comprises a backplate electrode12 and a second backplate electrode 14 being arranged in parallel toeach other. Some implementation examples (e.g., so called singlebackplate microphones) may not comprise the second backplate electrode14, i.e., the second backplate electrode 14 may be optional. Thebackplate electrode 12 may also be referred to as the top backplateelectrode. The second backplate electrode 14 may also be referred to asthe bottom backplate electrode. The attributes “top” and “bottom” mainlyserve to distinguish the two backplate electrodes 12 and 14 regardingtheir graphical representation in FIG. 1a and possibly further figures,and should not to be construed as limiting. In the following descriptionand figures, the index “T” typically refers to the top backplateelectrode 12, the index “B” typically refers to the bottom backplateelectrode 14, the index “M” typically refers to the membrane 16, and theindex “S” typically refers to the substrate 20_1, 20_2. The MEMS device10 further comprises a membrane 16. The membrane 16 is disposed betweenthe backplate electrode 12 and the second backplate electrode 14 inparallel. The membrane 16 can comprise a displaceable portion and afixed portion. The displaceable portion may be able to move in responseto an incident sound wave. For example, the displaceable portion may bedeflected or deformed. Alternatively, the displaceable portion may bedisplaced by a translation. The backplate electrode 12, second backplateelectrode 14 and membrane 16 can comprise electrical conductivematerial.

The backplate electrode 12, the second backplate electrode 14 and themembrane 16 may be supported by a support structure 18_1, 18_2. Thematerial of the support structure 18_1, 18_2 can be made of oxide. Thesupport structure 18_1, 18_2 itself may be supported on a substrate20_1, 20_2.

The backplate electrode 12 may comprise perforations to allow soundpressure to pass through the backplate electrode 12 to arrive at themembrane 16. The second backplate electrode 14 may be arranged on theside of the membrane 16 facing away from the direction of sound pressurearrival. Some of the air in the gap between the membrane 16 and thesecond backplate 14 may be pushed by the membrane 16 when moved as aconsequence of sound pressure arrival. To allow the volume between themembrane 16 and the second backplate electrode 14 to escape, the secondbackplate electrode 14 may be provided with perforations as well. Abackside cavity 22 may be provided allowing the air volume to expandwhich is pushed by the membrane 16.

The MEMS device 10 may suffer from parasitic capacitances created insidethe support structure 18_1, 18_2. As shown in FIG. 1a , a firstparasitic capacitance C_(TM) may be observed in at least a portion ofthe support structure 18_1, 18_2 between the backplate electrode 12 andthe membrane 16. A second parasitic capacitance C_(MB) may be observedin at least a portion of the support structure 18_1, 18_2 between themembrane 16 and the second backplate electrode 14. A further parasiticcapacitance C_(BS) may be observed in at least a portion of the supportstructure 18_1, 18_2 between the second backplate electrode 14 and thesubstrate 20_1, 20_2. While not mentioned, further parasiticcapacitances can be observed in specific portions of the supportstructure 18_1, 18_2 between different ones of the backplate electrode12, the membrane 16 and the second backplate electrode 14.

FIG. 1b shows a schematic circuit diagram of the MEMS device depicted inFIG. 1a . The substrate may be grounded. The membrane may be biased by abias voltage V_(bias). A voltage level Vp of the electrically conductivetop backplate electrode 12 may be output. Further, a voltage level Vm ofthe electrically conductive (bottom) second backplate electrode 14 maybe output. The voltage level outputs Vp and Vm vary in relation to themovement of the voltage biased membrane 16.

In other words, the MEMS device may be able to produce output signals Vpand Vm responsive to the movement of the membrane in relation to thebackplate electrode and the second backplate electrode (refer to FIG. 1a). This feature is schematically indicated by variable capacitors CA andCB, also referred to as active capacitances CA and CB. The activecapacitances CA, CB are variable in relation to the movement of themembrane in relation to the backplate electrode and the second backplateelectrode, respectively.

The circuit diagram shown in FIG. 1b further indicates parasiticcapacitances C_(TM), C_(MB) and C_(BS) observed in specific portions ofthe support structure between different ones of at least the backplateelectrode 12, the membrane 16, the second backplate electrode 14 and thesubstrate 20_1, 20_2.

FIG. 2a shows a schematic diagram of a MEMS device 10 in across-sectional view. This schematic diagram differs from the schematicdiagram shown in FIG. 1a in that further parasitic capacitances C_MS andC_BT may be observed inside the support structure 18_1 and 18_2. Theparasitic capacitance C_MS may be observed inside portions of thesupport structure 18_1 disposed between the membrane 16 and thesubstrate 20_1. The parasitic capacitance C_BT may be observed betweenthe second backplate electrode 14 and the backplate electrode 12 inportions of the support structure 18_2 disposed between the secondbackplate electrode 14 and the backplate electrode 12. Variable activecapacitances CA and CB are observed between the membrane 16 and thebackplate electrode 12 and the second backplate electrode 14,respectively.

FIG. 2b shows a schematic circuit diagram of the MEMS device 10comprising the further parasitic capacitances C_MS, C_BT. In thiscircuit diagram, the parasitic capacitance C_MS may be observed betweenthe input bias voltage V_(bias) and ground gnd. Further, the parasiticcapacitance C_BT may be observed between the voltage level outputs Vpand Vm. Variable active capacitances CA and CB are observed between themembrane and the backplate electrode and the second backplate electrode,respectively (refer to FIG. 2a ). The membrane is movable in relation toboth the backplate electrode and the second backplate electrode. Hence,active capacitances CA and CB vary in relation to this membranemovement, generated by sound pressure arrival, etc., for example.

Referring back to FIG. 2a and in comparison with FIG. 1a , the supportstructure 18_1 may comprise a portion disposed between the membrane 16and the substrate 20_1 having no second backplate electrode 14interposed. Hence, in said portion the parasitic capacitance C_MS may beprovided between the membrane 16 and the substrate 20_1 directly.Further, the support structure 18_2 may comprise a portion between thebackplate electrode 12 and the second backplate electrode 14 having nomembrane 16 interposed. Hence, in said portion the parasiticcapacitances C_BT may be provided between the backplate electrode 12 andthe second backplate electrode 14 directly.

FIG. 3a shows a plan view of a MEMS device 10, and FIG. 3b shows theMEMS device 10 in cross-section in a schematic view. In contrast to theschematic view depicted in FIG. 2a , the backplate electrode 12, themembrane 16 and the second backplate electrode 14 may have allsubstantially same dimensions and may be stacked in relation to eachother substantially without any offset. In other words, the backplateelectrode 12, the membrane 16 and the second backplate electrode 14 maybe arranged such to achieve maximum overlapping. This arrangement can beregarded as a worst case situation, since parasitic capacitances C_TM,C_MB and C_BS may be high compared to the arrangements depicted in FIGS.1a and 2a . For example, parasitic capacitance C_TM may be about 1.0 pF,parasitic capacitance C_MB may be about 1.0 pF and parasitic capacitanceC_BS may be about 4.1 pF. Since the membrane 16 is fully disposedbetween the backplate electrode 12 and the second backplate electrode14, the parasitic capacitance C_BT can be regarded as negligible.Further, since the second backplate electrode 14 is disposed fullybetween the membrane 16 and the substrate 20_1, 20_2, the parasiticcapacitance C_MS can be regarded as negligible, too.

FIG. 4a shows a MEMS device 10 in a plan view, and FIG. 4b shows aschematic diagram of the MEMS device 10 depicted in FIG. 4a in across-sectional view. In this arrangement, as can be best seen in FIG.4a , the circumference of the backplate electrode 12 may compriserecesses 24. The recesses 24 may be arranged along the circumference ofthe backplate electrode 12 in an equidistant manner. While not shown inFIG. 4a , also the second backplate electrode can comprise acircumference provided with recesses, wherein the recesses can bearranged along the circumference in an equidistant manner (oruniformly), too. Further, while not shown in FIG. 4a , also the membrane16 can comprise a circumference provided with recesses, wherein therecesses can be arranged along the circumference in an equidistantmanner, as well. The recesses mentioned above are adapted to reduce theoverlapping area in relation to each other.

Referring to FIG. 4b , this arrangement is likely to achieve reducedparasitic capacitances C_TM, C_MB and C_BS as well as reduced parasiticcapacitances C_BT and C_MS, compared to parasitic capacitances createdin the support structure of MEMS devices schematically shown in FIGS.1a, 2a and 3b . For example, parasitic capacitance C_TM is 0.12 pF,parasitic capacitance C_MB is 0.12 pF and parasitic capacitance C_BS is0.82 pF.

Compared to the arrangement shown in FIG. 3b , these parasiticcapacitances are all reduced. In the MEMS device 10 as shown in FIG. 4b, in the support structure 18_2, portions of the backplate electrode 12and the second backplate electrode 14 may be overlapped, without havingthe membrane 16 interposed. Hence, in this portion of the supportstructure 18_2 between the backplate electrode 12 and second backplateelectrode 14, parasitic capacitance C_BT is created. However, thisparasitic capacitance C_BT is reduced when compared to the arrangementshown in FIG. 3b , for example. In the arrangement shown in FIG. 4b , inthe support structure 18_1, a portion of the membrane 16 projects beyondthe backplate electrode 12 and second backplate electrode 14. In thisconfiguration, parasitic capacitance C_MS is created in such portions ofthe support structure 18_1 having no second backplate electrode 14interposed. However, this parasitic capacitance C_MS may be as small as0.25% C_BS in some implementation examples.

FIG. 5a shows a MEMS device 10 in a plan view, and FIG. 5b shows aschematic diagram of the MEMS device 10 depicted in FIG. 5a in across-sectional view. In this arrangement, as can be best seen in FIG.5a , the circumference of the backplate electrode 12 may compriserecesses 24. The recesses 24 may be arranged along the circumference ofthe backplate electrode 12 uniformly or in an equidistant manner.Compared to the arrangement shown in FIG. 4a , the recesses 24 may bestructured to comprise a larger radius of curvature. While not shown inFIG. 5a , also the second backplate electrode can comprise acircumference provided with recesses, wherein the recesses can bearranged along the circumference uniformly or in an equidistant manner.Further, while not shown in FIG. 5a , also the membrane can comprise acircumference provided with recesses, wherein the recesses can bearranged along the circumference in an equidistant manner.

Referring to FIG. 5b , this arrangement may provide reduced parasiticcapacitances C_TM, C_MB and C_BS as well as reduced parasiticcapacitances C_BT and C_MS compared to parasitic capacitances created inthe support structure of MEMS devices schematically shown in FIGS. 1a,2a, 3b and 4b . For example, parasitic capacitance C_TM may be about0.05 pF, parasitic capacitance C_MB may be about 0.05 pF and parasiticcapacitance C_BS may be about 0.3 pF.

Compared to the arrangement shown in FIG. 3b , for example, theseparasitic capacitances may be reduced. In the MEMS device 10 as shown inFIG. 5b , in the support structure 18_2, portions of the backplateelectrode 12 and the second backplate electrode 14 may overlap, withouthaving the membrane 16 interposed. Hence, in this portion of the supportstructure 18_2 between the backplate electrode 12 and second backplateelectrode 14, parasitic capacitance C_BT is created. However, thisparasitic capacitance C_BT can be considered negligible. In thearrangement shown in FIG. 5b , in the support structure 18_1, a portionof the membrane 16 may project beyond the backplate electrode 12 and thesecond backplate electrode 14. In this configuration, parasiticcapacitance C_MS may be created in such portions of the supportstructure 18_1 having no second backplate electrode 14 interposed.However, this parasitic capacitance C_MS may be as small as about 0.25%C_BS.

FIG. 6 shows a MEMS device 10 in a cross-sectional view. In general,this MEMS device 10 has substantially the same arrangement as comparedto the MEMS devices depicted in FIGS. 4b and 5b . However, the MEMSdevice 10 shown in FIG. 6 comprises a backplate electrode 12 and asecond backplate electrode 14 which comprise segmentations 12S and 14S.In particular, the backplate electrode 12 may comprise a segmentation12S. Further, the second backplate electrode 14 may comprise asegmentation 14S. The MEMS device 10 further may comprise a membrane 16disposed spaced apart from the backplate electrode 12 and the secondbackplate electrode 14. The membrane 16 may comprise a displaceableportion and a fixed portion. The fixed portion may be defined tocomprise at least a portion of the membrane 16 fixed by the supportstructure 18_1, 18_2. The displaceable portion may be defined tocomprise a portion of the membrane 16 being deflectable.

In the backplate electrode 12, the segmentation 12S may be arranged toprovide an electrical isolation between an active backplate portion12ABP and a further backplate portion 12FBP, the active backplateportion 12ABP facing the displaceable portion of the membrane 16. In thesecond electrode 14, the segmentation 14S is arranged to provide anelectrical isolation between an active backplate portion 14ABP and afurther backplate portion 14FBP, the active backplate portion 14ABPfacing the displaceable portion of the membrane 16.

Variable active capacitances CA and CB are observed between thedisplaceable portion of the membrane 16 and the active backplate portion12ABP of the backplate electrode 12 and the active backplate portion14ABP of the second backplate electrode 14, respectively. Hence, activecapacitances CA and CB vary in relation to the movement of thedisplaceable portion of the membrane 16 in relation to both the activebackplate portion 12ABP of the backplate electrode 12 and the activebackplate portion 14ABP of the second backplate electrode 14,respectively. This membrane 16 movement is generated by, for example,sound pressure arrival caused by speech, etc.

In case the MEMS device 10 comprises the backplate electrode 12, themembrane 16 and the second backplate electrode 14 having a circularshape, also the segmentations 12S and 14S may be formed circularly. Thesegmentations 12S and 14S may be arranged to extend in the vicinity ofthe support structure 18_1, 18_2 in order to segment the backplateelectrode 12 and the second backplate electrode 14 into the activebackplate portions 12ABP, 14ABP and the further backplate portion 12FBP,14FBP. Summarized, the active backplate portions 12ABP and 14ABP may becentral portions (medium portions) and the further backplate portions12FBP and 14FBP may be fringe portions of each the backplate electrode12 and the second backplate electrode 14.

The backplate electrode 12 and second backplate electrode 14 may be madeof, or comprise, an electrically conductive material, for examplepoly-silicon. By the provision of the segmentations 12S, 14S, theparasitic capacitances may be reduced greatly since the isolated(separated) further backplate portions 12FBP, 14FBP do not contribute tothe creation of parasitic capacitances. Each of the segmentations 12S,14S can be regarded as a fringing capacitance C_F which limits theparasitic coupling. Hence, the capacitance of the MEMS device 10 as awhole may comprise an active capacitance CA created between the activebackplate portion 12ABP of the backplate electrode 12 and thedisplaceable portion of the membrane 16, as well as an activecapacitance CB created between the active backplate portion 14ABP of thesecond backplate electrode 14 and the displaceable portion of themembrane 16.

Note, that both the active backplate portion 12ABP of the backplateelectrode 12 as well as the active backplate portion 14ABP of the secondbackplate electrode 14 may be connected to signal outputs via electricalconductive lead-throughs (not shown), respectively. The furtherbackplate portion 12FBP of the backplate electrode 12 may be isolated inthe immediate vicinity. Further, the further backplate portion 14FBP ofthe second backplate electrode 14 may be isolated in the immediatevicinity. Hence, parasitic capacitances C_TM, C_MB and C_BS may besubstantially eliminated. Further, parasitic capacitances C_MS and C_BTmay be substantially eliminated, too.

Both, the backplate electrode 12 as well as the second backplateelectrode 14 may comprise bonding layers 12B, 14B, respectively. Thebonding layers 12B, 14B may be arranged to support the active backplateportions 12ABP and 14ABP to the further backplate portions 12FBP and14FBP. The bonding layers 12B, 14B may be arranged to pull the backplateelectrode 12 and second backplate electrode 14 flat.

The material of the bonding layers 12B, 14B may comprise a dielectricmaterial. For example, the dielectric material may comprise SiN. Thebonding layers 12B, 14B may be bonded to surfaces of the backplateelectrode 12 and the second backplate electrode 14, respectively, facingeach other. This feature may provide a symmetrization. In anotherexample, the bonding layers 12B, 14B can be bonded to surfaces of thebackplate electrode 12 and the second backplate electrode 14,respectively, facing opposite directions. This feature may provide asymmetrization, as well. It may also be possible that the backplateelectrode 12 is bonded to a surface of the bonding layer 12B, and thatthe second backplate electrode 14 is bonded to a surface of the bondinglayer 14B.

FIG. 7 shows a plan view of the backplate electrode 12 or 14 (backplateelectrode 12 or second backplate electrode 14). This backplate electrode12 or 14 may comprise the active backplate portion 12ABP or 14ABP(medium portion) surrounded by the further backplate portion 12FBP or14FBP (fringe portion). Both portions may be isolated (separated) fromeach other via the segmentation 12S or 14S. A bonding layer 12B or 14Bmay be provided to bond the active backplate portion 12ABP or 14ABP tothe further backplate portion 12FBP or 14FBP which may be itselfsupported circumferentially by means of a support structure (not shown).A lead-through 26 may be provided which connects the active backplateportion 12ABP or 14ABP to a pad 28 serving as a contact point. Due tothe segmentation 12S or 14S, the further backplate portion 12FBP or14FBP may be completely isolated from the pad 28 and vice versa. Thebackplate electrode 12 or 14 may be perforated to allow transmission ofsound pressure to the membrane (not shown) in case the backplateelectrode is embodied as top backplate electrode.

FIG. 8a shows a MEMS device 10 in a plan view and FIG. 8b shows the MEMSdevice 10 in a cross-sectional view. In this arrangement, thecircumferences of the backplate electrode 12, the membrane 16 and thesecond backplate electrode 14 may be formed such to comprise recessesadapted to, in the support structure, reduce overlapping areas inrelation to each other. The recesses are arranged along thecircumferences in an equidistant manner.

Further, the backplate electrode 12, the second backplate electrode 14and the membrane 16 may comprise the same number of recesses.Furthermore, the recesses of the top backplate electrode 12, the bottombackplate electrode 14 and the membrane 16, respectively, may bearranged such to be offset to each other angularly.

FIG. 8b shows a cross-sectional view of the MEMS device 10. In thisconfiguration, the backplate electrode 12, the membrane 16 and thesecond backplate electrode 14 may comprise recesses. This configurationallows for reduced parasitic capacitances. In particular, the parasiticcapacitance C_TM, created inside the support structure 18_1 (i.e., thecapacitance between the fixed portion of the membrane 16 and thecorresponding portion of the top backplate electrode 12) equals about0.0 pF, since the backplate electrode 12 and the membrane 16 do notoverlap inside the support structure 18_1. Further, the parasiticcapacitance C_MB created inside the support structure 18_1 equals about0.0 pF, since the membrane 16 and the second backplate electrode 14 donot overlap inside the support structure 18_1, as well. In other words,the fixed portion of the membrane 16 does not (or only little) overlapwith the corresponding portion of the second backplate electrode 14. Theportions of the backplate electrode 12 and of the second backplateelectrode 14 that correspond to the fixed portion of the membrane 16 maybe regarded as the clamped portions of the backplate electrode 12, 14,as opposed to their exposed portions which may correspond substantiallyto the displaceable portion of the membrane 16. Further, the parasiticcapacitance C_BS created between the second backplate electrode 14 andthe substrate 20_1 equals about 0.58 pF.

In addition, the parasitic capacitance C_BT created between thebackplate electrode 12 and the second backplate electrode 14 may benegligible. Further, the parasitic capacitance C_MS may equal about 25%of C_BS. Furthermore, the parasitic capacitance C_TS created between thebackplate electrode 12 and the substrate 20_2, inside the supportstructure 18_2, may equal about 14% of C_BS. Hence, parasiticcapacitances typically may be reduced significantly.

For a better overview, FIG. 8c shows the angular offset arrangementbetween the backplate electrode 12, the second backplate electrode 14and the membrane 16 in a schematic view. In this case, the secondbackplate electrode 14 is offset from the backplate electrode 12 by anangular offset of 30°. Further, the membrane 16 is offset from thesecond backplate electrode 14 by an angular offset of 30°.

Due to this arrangement the backplate electrode 12, the second backplateelectrode 14 and the membrane 16 may be arranged such that in an areacorresponding to the fixed portion (i.e., inside the support structure)of the membrane 16 the overlapping area in relation to each otherexhibits substantially minimum overlapping. Further, the membrane 16 maybe arranged such that in the fixed portion the overlapping area inrelation to both the backplate electrode 12 and second backplateelectrode 14 may be less than maximum overlapping. Furthermore, theangular offsets of the recesses of the backplate electrode 12, thesecond backplate electrode 14 and the membrane 16 may comprise a valueresulting in minimum overlapping with each other. Furthermore, thebackplate electrode 12, the second backplate electrode 14 and themembrane 16 may be arranged such that in an area corresponding to thedisplaceable portion of the membrane 16 (e.g., central portion of themembrane) the overlapping area in relation to each other exhibitssubstantially maximum overlapping.

The recesses can be shaped to have a semi-circular shape, a circlesegment shape, a castellation shape or other shape.

FIG. 9a shows a schematic diagram of a MEMS device 10 in across-sectional view. Similar to the arrangements shown in FIGS. 4b, 5b, 6 and 8 b, the backplate electrode 12, the membrane 16 and the secondbackplate electrode 14 may be arranged such that an overlapping area ofthe fixed portion of the membrane 16 with the backplate electrodes 12,14 is less than maximum overlapping. In the MEMS device 10 in thearrangement shown in FIG. 9a , a guard ring 30 may be interposed betweenthe substrate 20_1, 20_2 and the support structure 18_1, 18_2. The guardring 30 may be associated with the second backplate electrode 14 andadapted to reduce parasitic capacitance in the support structure 18_1,18_2 (i.e., fixed portion of the membrane 16 with the backplateelectrodes 12, 14). In particular, the guard ring 30 may be adapted toreduce parasitic capacitances between the second backplate electrode 14and the guard ring 30 itself. The guard ring 30 may be interposedcircumferentially.

Further, as depicted in FIG. 9b , additional guard rings 30_1 to 30_3can be provided, wherein each guard ring is associated with thebackplate electrode 12, the membrane 16 and the second backplateelectrode 14, respectively. In particular, each guard ring 30_1 to 30_3may be associated with one protruding arm of the backplate electrode 12,the membrane 16, or the second backplate electrode 14, wherein eachprotruding arm may mechanically connect a corresponding central portion(e.g., a suspended portion) of the backplate electrode 12, the membrane16, and the second backplate electrode 14, respectively, with thesupport structure. In this case, each guard ring 30_1 to 30_3 may beassociated with the respective one of the backplate electrode 12, themembrane 16 and the second backplate electrode 14. Due to the provisionof the guard rings 30_1 to 30_3, large reductions in parasiticcapacitances can be achieved.

FIGS. 10a to 10p show schematic cross-sections associated during variousstages or steps of an example manufacturing process of a MEMS device asdescribed above.

FIG. 10a shows the substrate 100 which can be made of a silicon wafer.

As shown in FIG. 10b , a lower etch stop layer 102 is deposited onto theupper surface of the substrate 100. The lower etch stop layer 102 mayprovide a reliable stop of an etching process. The lower etch stop layer102 may typically be made from a stop oxide TEOS (tetraethylorthosilicate). The thickness of the lower etch stop layer 102 may betypically about 600 nm.

FIG. 10c shows a schematic cross-sectional view of the arrangement upondepositing a SiN layer 104 onto the surface of the lower etch stop layer102 and upon depositing a poly-silicon layer 106 onto the surface of thedeposited SiN layer 104. The SiN layer 104 may have a thickness of about140 nm. The poly-silicon layer 106 may have a typical thickness of about330 nm.

FIG. 10d shows a schematic cross-sectional view after a multilayerarrangement comprising the three layers 102, 104 and 106 have beendeposited onto the substrate 100 and the poly-silicon layer 106 has beenstructured, for example by etching, to provide a poly-silicon layer 106segmentation.

FIG. 10e shows a schematic cross-sectional view after a multilayerarrangement comprising the SiN layer 104, poly-silicon layer 106 and afurther SiN layer 108 has been structured. In particular, openings ortrenches may be formed in the multilayer arrangement, wherein saidopenings extend to the oxide layer 102.

FIG. 10f shows a schematic cross-section after filling the wholesubstrate 100 surface with a TEOS deposition layer 110 by means of adeposition process, for example. In particular, the TEOS depositionlayer 110 may have been filled into the openings. Afterwards, the TEOSlayer 110 may have been annealed. A chemical mechanical polishing (CMP)process may have been applied to the TEOS layer 110, subsequently.

FIG. 10g shows a schematic cross-sectional view of the arrangement aftera further TEOS layer 112 may have been applied on top of the polishedTEOS layer 110. Afterwards, this TEOS layer 112 may have also beenannealed.

FIG. 10h shows a schematic cross-sectional view of the arrangement afterthe additional TEOS layer 112 may have been subjected to a process offorming recesses 114. The recesses 114 may be used for a subsequentformation of anti-sticking bumps. The anti-sticking bumps may bearranged to reduce the risk that a membrane element (not shown) getsstuck to an underlying electrode due to an adhesive force. Theanti-sticking bumps 114 may be formed by etching, for example.

FIG. 10i shows a schematic cross-sectional view of the arrangement aftera membrane layer 116 may have been deposited onto the TEOS layer 112. Inparticular, the membrane layer 116 may have been deposited onto the TEOSlayer 112 to fill the recesses 114 in order to form respectiveanti-sticking bumps. The membrane layer 116 may have a thickness ofabout 330 nm and comprises poly-silicon, for example.

FIG. 10j shows a schematic cross-sectional view of the arrangement aftera TEOS layer 118 may have been deposited on top of the TEOS layer 112 aswell as on top of the membrane layer 116. After deposition of the TEOSlayer 118, this layer is etched to form recesses 120 to be used for asubsequent formation of anti-sticking bumps in the (top) backplateelectrode 12.

FIG. 10k shows a schematic cross-sectional view of the arrangement aftera bonding layer 122 may have been deposited on top of the TEOS layer118. The bonding layer 122 may comprise SiN. The bonding layer 122 maybe deposited such to fill the whole surface as well as the recesses 120for obtaining respective anti-sticking bumps (refer to FIG. 10j ). Afterthis process, a poly-silicon layer 124 may be deposited on top of theSiN layer 122. Subsequently, the poly-silicon layer 124 may besegmented, for example, by etching.

FIG. 10l shows a schematic cross-sectional view of the arrangement afterdeposition of a SiN layer 126 on top of the arrangement shown in FIG.10k . In particular, the SiN layer 126 may be deposited on top of thepoly-silicon layer 124. After deposition of the SiN layer 126, thislayered arrangement comprising the SiN layer 122, the poly-silicon layer124 and the SiN layer 126 may be etched to form a plurality of openingsor trenches, which extend to the TEOS layer 118.

FIG. 10m shows a schematic cross-sectional view of the arrangement afterdeposition of a TEOS layer 128 on top of the layered arrangementcomprising the SiN layer 122, the poly-silicon layer 124 and the SiNlayer 126 as mentioned above. The TEOS layer 128 has a thickness of 100nm, for example.

Subsequent to this process, a contact hole 130 may be formed by means ofphotolithography. This contact hole 130 may be formed to extend acrossthe SiN layer 126 and the TEOS layer 128 until reaching the poly-siliconlayer 124. Further, as shown on the right-hand side of FIG. 10m , firstto third trenches 132, 134 and 136 may be formed such to extend acrossrespective TEOS layers of the TEOS layers 110, 112, 118.

In particular, the first trench 132 may be formed to extend across theTEOS layer 118 such to reach the membrane layer 116 (refer to FIG. 10i). Further, the second trench 134 may be formed to extend across theTEOS layers 118 and 112 such to reach the poly-silicon layer 106 (referto FIG. 10e ). Further, the third trench 136 may be formed to extendacross the TEOS layers 110, 112 and 118, as well as the lower etch stoplayer 102 such as to reach the substrate 100.

FIG. 10n shows a schematic cross-sectional view of the arrangement afterfilling of electrically conductive material into the contact hole 130and into the first to third trenches 132, 134 and 136. In doing so,contacts 138, 140, 142 and 144 may be created, connected tosurface-mounted pads 146, 148, 150 and 152, respectively.

In particular, the contacts 138, 140, 142 and 144 may be formed suchthat the respective contact pads 146, 148, 150 and 152 may beelectrically connected to the respective layers, respectively, i.e., thepoly-silicon layer 124, the membrane layer 116, the poly-silicon layer106 and the substrate 100. In general, connections may be created toachieve electrical connections to the second backplate electrode, themembrane, the backplate electrode and the substrate, respectively. Thematerial of the contacts 138, 140, 142 and 144 can be titanium,platinum, or gold, for example. Of course, other materials having goodelectrical conductivity can be selected.

FIG. 10o shows a schematic cross-sectional view of the MEMS device 10after backside etching of a backside cavity 154. In this etching step,the lower etch stop layer 102 may act as an etch stop element preventingthe etching agent to reach the second backplate electrode.

FIG. 10p shows a schematic cross sectional view of the MEMS device 10after removal of parts of the TEOS layers 110, 112 and 118 insideoperation portions of the backplate/membrane/backplate-arrangement. Thisstep may be assisted by a front side protection mask opening theperforations of the backplate electrode.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus.

The above described is merely illustrative, and it is understood thatmodifications and variations of the arrangements and the detailsdescribed herein will be apparent to others skilled in the art. It isthe intent, therefore, to be limited only by the scope of the impendingclaims and not by the specific details presented by way of descriptionand explanation above.

What is claimed is:
 1. A MEMS device comprising: a backplate electrode;and a membrane spaced apart from the backplate electrode, wherein themembrane comprises a displaceable portion and a fixed portion; whereinthe backplate electrode and the membrane are arranged such that anoverlapping area of the fixed portion of the membrane with the backplateelectrode is less than maximum overlapping; and wherein the backplateelectrode comprises a segmentation providing an electrical isolationbetween an active backplate portion and a further backplate portion, theactive backplate portion facing the displaceable portion of themembrane.
 2. The MEMS device according to claim 1, wherein the activebackplate portion is a medium portion and wherein the further backplateportion is a fringe portion.
 3. The MEMS device according to claim 1,wherein the backplate electrode comprises a bonding layer that pulls thebackplate electrode flat.
 4. The MEMS device according to claim 3,wherein the bonding layer comprises a dielectric material.
 5. The MEMSdevice according to claim 3, further comprising a second backplateelectrode at an opposite side of the membrane than the backplateelectrode, the second backplate electrode comprising a second bondinglayer, wherein the bonding layer and the second bonding layer are bondedto surfaces of the backplate electrode and second backplate electrode,respectively, facing each other or facing opposite directions.
 6. A MEMSdevice comprising: a backplate electrode; a membrane spaced apart fromthe backplate electrode, wherein the membrane comprises a displaceableportion and a fixed portion; and a second backplate electrode at anopposite side of the membrane than the backplate electrode, wherein thebackplate electrode and the membrane are arranged such that anoverlapping area of the fixed portion of the membrane with the backplateelectrode is less than maximum overlapping; the backplate electrode andthe second backplate electrode are arranged such that in an areacorresponding to the fixed portion of the membrane the overlapping areain relation to each other comprises maximum overlapping; and themembrane is arranged such that in the fixed portion the overlapping areain relation to both the backplate electrode and second backplateelectrode is less than maximum overlapping.
 7. The MEMS device accordingto claim 1, wherein a circumference of the backplate electrode or themembrane comprises recesses adapted to reduce the overlapping area inrelation to each other.
 8. The MEMS device according to claim 7, whereinthe recesses have a semi-circular shape, a circle segment shape, or acastellation shape.
 9. The MEMS device according to claim 7, wherein therecesses are arranged along the circumference in an equidistant manner.10. The MEMS device according to claim 9, wherein the backplateelectrode and the membrane comprise a same number of recesses and arearranged such that the recesses are offset to each other angularly. 11.The MEMS device according to claim 10, wherein the angular offsets ofthe recesses of the backplate electrode and the membrane comprise avalue resulting to minimum overlapping to each other.
 12. A MEMS devicecomprising: a backplate electrode; a membrane spaced apart from thebackplate electrode, wherein the membrane comprises a displaceableportion and a fixed portion, wherein the backplate electrode and themembrane are arranged such that an overlapping area of the fixed portionof the membrane with the backplate electrode is less than maximumoverlapping; a second backplate electrode at an opposite side of themembrane than the backplate electrode; a substrate adapted to support asupport structure; and a guard ring interposed between the substrate andthe support structure, the guard ring being associated with the secondbackplate electrode and adapted to reduce parasitic capacitances in thesupport structure.
 13. The MEMS device according to claim 12, whereinthe guard ring is interposed circumferentially and adapted to reduceparasitic capacitances between the second backplate electrode and thesubstrate.
 14. The MEMS device according to claim 12, further comprisingadditional guard rings arranged in the support structure, the additionalguard rings associated with the backplate electrode and membrane,respectively.
 15. The MEMS device according to claim 1, wherein thebackplate electrode and the membrane each comprise electricallyconductive material.
 16. An electrostatic transducer comprising: abackplate electrode; and a membrane spaced apart from the backplateelectrode, wherein the membrane comprises a displaceable portion and afixed portion; wherein the backplate electrode and the membrane arearranged such that an overlapping area of the fixed portion of themembrane with the backplate electrode is less than a maximumoverlapping; wherein the electrostatic transducer is configured toproduce an output signal responsive to a movement of the membrane inrelation to the backplate electrode; wherein the backplate electrodecomprises a segmentation providing an electrical isolation between anactive backplate portion and a further backplate portion, the activebackplate portion facing the displaceable portion of the membrane; andwherein the active backplate portion is a central portion and whereinthe further backplate portion is a peripheral portion.
 17. A method formanufacturing a MEMS device that comprises a backplate electrode and amembrane spaced apart from the backplate electrode, wherein the membranecomprises a displaceable portion and a fixed portion, the methodcomprising: assembling the backplate electrode and the membrane suchthat an overlapping area of the fixed portion of the membrane with thebackplate electrode is less than a maximum overlapping; and wherein thebackplate electrode is provided with a segmentation providing anelectrical isolation between an active backplate portion and a furtherbackplate portion, the active backplate portion facing the displaceableportion of the membrane.
 18. A method for manufacturing a MEMS devicethat comprises a backplate electrode and a membrane spaced apart fromthe backplate electrode, wherein the membrane comprises a displaceableportion and a fixed portion, the method comprising: assembling thebackplate electrode and the membrane such that an overlapping area ofthe fixed portion of the membrane with the backplate electrode is lessthan a maximum overlapping; providing a second backplate electrode at anopposite side of the membrane than the backplate electrode; providingthe backplate electrode and the second backplate electrode such that inan area corresponding to the fixed portion of the membrane theoverlapping area in relation to each other comprises maximumoverlapping; and providing the membrane such that in the fixed portionthe overlapping area in relation to both the backplate electrode and thesecond backplate electrode is less than maximum overlapping.
 19. A MEMSdevice comprising: a backplate electrode, a circumference of thebackplate electrode comprising first recesses; a membrane spaced apartfrom the backplate electrode, a circumference of the membrane comprisingsecond recesses, wherein the membrane comprises a displaceable portionand a fixed portion; and a support structure radially spaced apart froma central portion of the backplate electrode by a first distance andradially spaced apart from a central portion of the membrane by a seconddistance equal to the first distance, wherein peripheral portions of thebackplate electrode and peripheral portions of the membrane aresupported by the support structure, and wherein the first recesses andthe second recesses are arranged such that the first recesses areangularly offset from the second recesses.
 20. The MEMS device accordingto claim 19, wherein the first recesses and the second recesses have asemi-circular shape, a circle segment shape, or a castellation shape.21. The MEMS device according to claim 19, wherein the first recessesand the second recesses are arranged along respective circumferences inan equidistant manner.
 22. The MEMS device according to claim 21,wherein the first recesses and the second recesses comprise a samenumber of recesses.
 23. The MEMS device according to claim 19, whereinthe angular offsets of the first recesses and the second recessescomprise a value resulting in minimum overlapping between the backplateelectrode and the membrane.